1. Field of the Invention
The invention relates to electro-optical modulator/deflectors.
2. Description of the Prior Art
Particular types of electro-optical modulators have been recently developed such as, for example, described in Electronic Letters 9, 1973, pages 309 and 310, and in Proceedings of IEE, Volume 199, No. 7, 1972, pages 807 to 814. The operation of these modulators depends on the effect of applying a voltage to a symmetrical electrode pattern to induce periodic change of the refractive index in an electro-optic element in the region of the surface of the element at the moment of total internal reflection. The resulting symmetrical periodic phase change induced in the wave-front of a light beam directed through or at the electro-optical element produces a far field pattern of the Raman-nath form exhibiting symmetry about the zero energy order. The zero order of the output beam can best be modulated by adjusting the applied voltage to the electrode pattern to alter the distribution of the light from the zero order into higher orders, and as often desired, to eliminate the zero order.
It has been noted that at zero applied voltage a residual-diffraction pattern appears in the far field. It has already been disclosed that this pattern is due to reflection of the light off the interdigital electrode pattern on the surface of the electro-optical element. That is, the electrode pattern produces an inherent diffraction grating effect in its own right before the application of the voltage to the electrodes. The presence of the residual-diffraction pattern appears as `noise` in practical configurations of electro-optical modulators.